Prior telephone switching control units, e.g., key telephone system controllers or private branch exchanges (PBXs), are not fully compatible with a configuration in which both so-called tip and ring devices (TR devices) available today and enhanced functionality or key type station sets (key sets) are bridged simultaneously to an individual interface port. In the context of this application the terms key sets and TR devices or sets are used in accordance with the modern understanding of these terms in the art. Accordingly, key sets are meant to be multifunction telephone station sets or station equipment type adjuncts that utilize special digital messaging protocols to communicate with the control unit concerning actions taken at the set and/or actions that the set should take. TR devices are the well known tip and ring devices including, without limitation: standard, home-use type telephone sets--both rotary and multi-frequency signaling types; answering machines; facsimile (FAX) machines; and modems. The incompatibility results from the difference in nature and format between the information required to support the functionality of a key set and the information required to support the functionality of a TR device. To allow the use of both key sets and TR devices with the same control unit in a key telephone system some implementations utilize one set of interface boards to connect to TR devices and another set of interface boards to connect to key sets. Alternative implementations provide an interface only for key type sets and require expensive adaptors to utilize lower cost commercially available tip and ring devices. Prior systems lacked the ability to alert and control TR devices and a key set independently at any port or simultaneously bridged onto the same port. Consequently, the features and call coverage options available to users of these priors system was limited. This increased the cost of these prior systems while limiting their usefulness.
One commercially available series of telephone systems will accept both key telephone sets and TR sets in a bridged configuration to a single port despite their incompatibility. In a such a configuration, however, only the key set is fully operated by the control unit. The TR device is not alerted by the control unit. Further, a TR device bridged in such a configuration can neither place calls nor operate features. Additionally, no method of controlling access to the voiceband communication medium shared by the bridged key set and TR device is provided. Accordingly, this system has noticeable drawbacks in many typical situations. For example, providing coverage with an answering machine of calls that are routed to a particular port requiring an enhanced functionality telephone necessitates either the use of an additional port or a special adaptor. Similarly, the use of a modem bridged with a key set would require placing the call by manual dialing from a key set. Also, a potential exists in such a configuration that data being supplied to or from the modem could be corrupted by voice information being transmitted from the key set. Also, this prior system does not recognize pulse (rotary) type dialing from a TR device in any configuration.